Nonvolatile semiconductor memory device for storing multivalued data

Abstract

A multivalued memory has data of state “0”, state “1”, state “2”, and state “3” whose threshold voltages increase in that order. In a first-page write operation, a memory cell whose data is in state “0” is brought into state “1”. In a second-page write operation, a memory cell whose data is in state “0” is brought into state “3” and a memory cell whose data is in state “1” is brought into state “2”. As a result, in reading the data, the data on the first page can be read in two read operations. Furthermore, the operation of writing the data onto the second page can be made faster, because a high initial write voltage can be used.

What is claimed is:  
     
       1. A method for programming a semiconductor memory device comprising: 
       loading a second data to a first latch circuit when data of a memory cell has been changed according to a first data;  
       loading the first data readout from the memory cell to a second latch circuit;  
       writing the second data latched in the first latch circuit to the memory cell;  
       verifying data read from the memory cell to check whether data set to state “1” according to the first data has reached state “2” according to the second data; and  
       verifying data read from the memory cell to check whether data set to state “0” according to the first data has reached state “3” according to the second data.  
     
     
       2. The method according to  claim 1 , further comprising: 
       loading the first data to the first latch circuit;  
       writing the first data to the memory cell; and  
       verifying whether data read from the memory cell has reached state “1” or still “0”,  
       wherein said loading the first data, writing the first data and said verifying being executed prior to loading the second data to the first latch circuit.  
     
     
       3. The method according to  claim 2 , wherein said verifying the data comprises reading data from the memory cell by a first potential which is higher than a potential of a read operation. 
     
     
       4. A method for programming a semiconductor memory device comprising: 
       loading a first data to a first latch circuit;  
       writing the first data to a memory cell;  
       firstly verifying whether data read from the memory cell has reached state “1” or still state “0”;  
       loading a second data to the first latch circuit when the data read from the memory cell has changed according to the first data;  
       loading the first data readout from the memory cell to a second latch circuit;  
       writing the second data to the memory cell;  
       secondly verifying data read from the memory cell to check whether data set to state “1” according to the first data has reached state “2”; and  
       thirdly verifying data read from the memory cell to check whether data set to state “0” according to the first data has reached state “3”.  
     
     
       5. The method according to  claim 4 , wherein said firstly verifying data comprises reading data from the memory cell by a first potential which is higher than a potential when reading data corresponding to state “1”. 
     
     
       6. The method according to  claim 4 , wherein said secondly verifying data comprises reading data from the memory cell by a second potential which is higher than a potential when reading data corresponding to state “2”. 
     
     
       7. The method according to  claim 4 , wherein said thirdly verifying data comprises reading data from the memory cell by a third potential which is higher than a potential when reading data corresponding to state “3”.